1. Field of the Invention
The present invention relates to methods of and apparatus for producing containers. More particularly, the invention relates to forming operations carried out on open-ended container bodies during the manufacture of containers, for example die necking, shaping (expansion), flanging, and similar forming operations.
2. Background Art
In the following discussion, reference is made to die-necking as an example of the type of forming operations that may be carried out, but it should be kept in mind that the present invention is not limited solely to die-necking operations.
The technology for modifying an open-ended portion of a closed end container has been in existence for over one hundred years. The procedure was originally developed for artillery shells, with a larger diameter shell casing being reduced at the open end to retain a smaller diameter projectile. The process by which this is accomplished today is called die-necking. The basic concept of necking is to take a typically cylindrical, thin walled metal container body or shell having a given diameter and physically push the open end into a die or series of progressively smaller dies. In the course of this process, a reduction in diameter of the open end is realized.
Nowadays, in the case of metal containers intended for food and beverages, a primary purpose for reducing the diameter at the open-end is material savings, and thus cost savings. Containers of this kind are normally produced by manufacturing an open-ended container body, and then closing the container body with an end panel. Because the end panel is normally of a thickness that is greater than the thickness of a typical sidewall, as the diameter of the container body is reduced at the open end, the amount of material required for the end panel is reduced by a greater amount. Alternatively, in certain other applications, such as aerosol containers, the necking operation is performed to bring the opening to a specific diameter to accommodate a standard size valve assembly and to eliminate any secondary adaptor that would otherwise be necessary to accommodate differences of size. A second consideration in such operations is the reduction in the longitudinal stress exerted on the end of the container when the container is filled with a substance under pressure. As the end wall size is reduced, the total force it encounters at a given pressure is reduced and the end wall can thus be reduced further in thickness. A third consideration for diameter reduction is visual. Many aesthetically pleasing container shapes can be achieved by necking conventional cylindrical shapes into tapered geometries and containers that resemble bottles.
There are practical limits to the reduction of the diameter of the open end of a container body for any given material achievable by any given die. The strength of the container body depends on a number of factors including the Young's modulus and yield stress of the material, the plate thickness and the container diameter. If the practical limit on diameter reduction is exceeded, the material will wrinkle, pleat, pucker or tear at a point inherent to the geometrical characteristics and type of metal being necked.
Conventional die necking of metal containers is accomplished with large-scale machinery, and it is very difficult to fine-tune the parameters necessary to manufacture containers with significant container neck length. The development of necking profiles is currently a long and involved process of trial and error that can require months to establish the proper parameters for each necking stage necessary to produce longneck containers. Specifically, current die necker technology uses hard cams to provide motion to pushers and knock-out rams. Key parameters such as the cam profile and cam throw must be tested and tweaked with each incremental change in the necking profile. Each time a change is made, the machine must be taken down and modified in a lengthy process to redesign and refit the new cams.
U.S. Pat. No. 5,355,710 discloses a conventional method and apparatus for necking a metal container. The disclosure of this patent is specifically incorporated herein by reference.
U.S. Pat. No. 5,768,931 discloses an apparatus having a number of planetary turrets that rotate on their own axes through a series of indexed positions. Tools engage a workpiece held in a planetary turret at successive workstations.
In modern manufacturing methods, it is important to provide apparatus that may carry out forming operations in a manner that is easily changeable, and that may be carried out with high throughput so that productivity is improved. Conventional methods have tended to be difficult to modify for new operations, and may be slow and temperamental. There is, therefore, a need for improved forming apparatus and methods.